Improving the synthesis of alkyl phosphates as sol-gel precursors
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Improving the synthesis of alkyl phosphates as sol-gel precursors Ashraf Fahim Ali,a) Piercarlo Mustarelli,b) Eliana Quartarone, and Aldo Magistris CSTE-CNR and Dipartimento de Chimica Fisica, Universit`a di Pavia, Via Taramelli 16, 27100 Pavia, Italy (Received 8 October 1997; accepted 8 September 1998)
Alkyl phosphates can be used as sol-gel precursors for phosphate-based glasses and glass-ceramics. An improvement in the synthesis of alkyl phosphates is presented. Following the procedure here described, it is possible to limit the fraction of condensed phosphates to 1–2%. At the same time, a ratio between mono- and di-substituted phosphates near to that given by the reaction stoichiometry may be attained.
Sol-gel phosphate glasses are generally obtained following the alkoxidic route.1,2 As the precursors, balanced mixtures of mono- and dialkyl phosphates are conveniently employed,3 which can be prepared following Cavalier’s reaction4 : 3ROH 1 P2 O5 ! sROd sOHd2 PO 1 sROd2 sOHdPO . (1) On the other hand, Eq. (1) must be considered quite approximate. In fact, phosphorus pentoxide is not a monomeric molecule, but it has the structural unit of at least P4 O10 , which consists of a close network of P–O–P bonds between the phosphoryl groups arranged approximately in a tetrahedron. The attack of an alcohol on such a structure proceeds by progressive cleavage of the anhydride bonds. When all the anhydride linkages of the “pentoxide unit” are destroyed, it is evident that formation of primary and secondary esters is the predominant process. However, there is also a minor probability that tertiary esters and phosphoric acid will be formed. In addition, the strong exothermic nature of these reactions may lead to alcohol evaporation. In the laboratory, the formation of pyro- and polyphosphates is the common result4 if we start from the stoichiometry of Eq. (1). We have recently shown by 31 P NMR that 15 –20% of condensed phosphates are the rule rather than the exception in alkyl phosphate preparation, also in the presence of well-controlled experimental conditions and long refluxing procedures.5 The presence of condensed phosphates, in turn, may slow down the gelation rate. It is a common practice, in precursor preparation, to use considerably greater molar quantities of alcohol in order to bring the solid P2 O5 into solution.3 On the a)
On leave from Department of Inorganic Chemistry, National Research Centre, Dokki, Cairo, Egypt. b) Address all correspondence to this author. J. Mater. Res., Vol. 14, No. 2, Feb 1999
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other hand, alcohol overstoichiometry leads to solvent effects such as reduction of the gelation rate,6 and causes deviations from the nominal ratio 1 : 1 between monoand dialkyl phosphates which is given by Eq. (1). In our previous paper5 we improved the preparation method suggested by Lee et al.7 However, also in our best case (ethyl phosphate) we found that ,24% of phosphorus atoms belonged to condensed uni
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